Change the value for "Vorlage" from "Symbol" to "Mehrere".
With "Mehrere", you are able to choose a style template from yEd's palette in the mapping details (the column "Symboldatei" will be replaced with a column "Vorlage").
With "Symbol", you are able to choose an icon/image/symbol file (e.g. an JPG, PNG, or SVG file) from your hard drive. The values in column "Symboldatei" have to be file system paths to the corresponding files.